Recently, however, units are frequently constructed which, for economic reasons, operate at elevated pressures, e.g. up to 25 or 50 bar. As experience with such units has shown, commercial iron oxide-chromium oxide catalysts lose their original mechanical strength relatively fast during the running time. This effect often causes the formation of fracture or dust and at the same time a definite increase in flow resistance.
Mechanical strength is generally understood to mean the lateral crushing strength (LSC), which is measured on catalyst tablets. With commercial iron oxide-chromium oxide catalysts, LCS losses of about 54 to 63% have been found after use in a high-temperature water-gas shift reaction plant at a pressure of 50 bar after a running time of 2000 hours. This applies also to catalysts with originally high LCS values.
Since under technical conditions the CO conversion takes place in the areas of the internal diffusion, both the porosity and the pore distribution in the catalyst compacts (in particular tablets) are of importance. The production of a catalyst with increased lateral compressive strength (15 kg/tablet) of the compacts requires the application of higher crushing pressures. This leads to a reduction of the pore volume as well as to an alteration of the pore distribution in the produced tablets and hence also to a considerable loss of apparent activity.